Tapered structure suitable for microthermocouples microelectrodes, field emission tips and micromagnetic sensors with force sensing capabilities

ABSTRACT

A tapered, nonconductive structure (10) having a hollow core (18) and a conical tip (14) includes a metal wire (16) within the core (18). The wire (16) is sealed within, and is exposed at the end (22) of, the tip (14). An electrically conductive or semiconductive layer (24) on the exterior of the tip (14) form a point thermocouple contact with the wire. The tip (14) may be fabricated, for example, by placing a metal wire (16) within a tube (12), and heating and pulling the tube (12) to produce two tapered micropipettes. Thereafter, a thin metal or semiconductor film (24) is evaporated onto the outer surface.

FIELD OF THE INVENTION

The present invention relates, in general, to the fabrication ofstraight and bent microelectrodes, microthermocouples, micromagnetic andfield emission tips for the measurement of spatially localized rapidtemperature changes, electrochemistry and micromagnetic inhomogeneitiesand the production of directed electron beams, and to the structure ofsuch tips. The disclosed methodology and structure have a wide varietyof uses, and allow for the interface of the device of the invention withall current scanned probe microscopes.

BACKGROUND OF THE INVENTION

The measurements of spatially localized rapid temperature changes arerequired in studies of many physical and biological processes andobjects. These can include such diverse subjects as turbulent flows,changes associated with processes of explosion and combustion,microtemperature measurements in biology at the cellular and subcellularlevel and microtemperature measurements in evolving chemical reactions.For all these applications a microthermocouple is the most convenientdetector.

In recent publications, fast microthermocouples have been described withresponse times of milliseconds and spatial resolutions of from hundreds[L. J. Forney, E. L. Meeks, J. Ma, and G. C. Fralick, Rev.Sci.Instrum.64, 1280 (1993)] to tens [P. Beckman, R. P. Roy, K. Whitfield, and A.Hasan, Rev.Sci.Instrum. 64, 2947 (1993)] of micrometers. Suchmicrothermocouples may also be used as point radiation microdetectors ina range of wavelengths from the UW to the IR.

In addition to the above Pendley and Abruna [B. D. Pendley and H. DlAbruna, Anal. Chem. 62, 782 (1990)] have considered the problem ofmicroelectrodes for microchemical measurements and achieved outerdiameters of a few microns under non-reproducible conditions. Inaddition, micromagnetic measurements are made using magnetic wires thatare electropolished in which it is difficult to construct such tips [K.Sueoka, F. Sai, K. Parker and J. Arnolddussen, J. Vac.Sci. and Tech.B12, 1618 (1994)]. Furthermore, there is great interest in makingmicrotip field emission tips [C. A. Spindt, et al. J. Appl.Phys. 47,5248 (1976)] but there is an active interest in new methodologies formaking such tips with better characteristics.

STATE OF PRIOR ART

No devices or methodologies have been reported that can provide thermalor electrochemical measurements in isolated submicrometer size volumesor in macrovolumes with submicrometer spatial resolution. In addition,the response time of such measurements are on the order of millisecondsand no shorter response times are known.

Recent publications, as noted above, have described response times ofmilliseconds with spatial resolutions from hundreds of micrometers. Inaddition, some recent publications have described thermal imaging usingatomic force microscopy (AFM) without specific measurements of the timeresponse of these devices [A. Majumdar, J. P. Carrejo and J. Lai, Appl.Phys. Lett. 62, 2501 (1993)]. The thermocouples that have been used inAFM employed a 2-wire thermocouple junction of 25μ diameter which endedin a sharp tip suitable for AFM. The two wire thermocouples that werefabricated acted as a thermal bridge in the measurements, distorting thereal thermal picture of the sample. These thermocouples reflected onlythe relative thermal character of the samples and did not give anabsolute thermal measure of the temperature.

In addition to the above, various attempts have been made to obtainmicroelectrodes for micro-electrochemical measurements. The mostsuccessful of these approaches was by Abruna [B. D. Pendley and H. D.Abruna, Anal. Chem 62, 782-784 (1990)]. However, the technology employedwas uncontrolled in the fabrication of these electrochemical probes, andlimited the dimensionality of the probes. Furthermore, it did not allowfor the interfacing of these electrodes with normal force sensingscanned probe microscopes.

Furthermore, micromagnetic measurements have only been performed withmagnetic wires and these are notoriously difficult to fabricate.Finally, field emission tips produced today by a variety of proceduresunrelated to the present invention have less than ideal characteristics.

SUMMARY OF THE INVENTION

The present invention provides a new technique for microtemperaturemeasurements with thermocouples that have a submicrometer contact sizeand a response time of a few microseconds. The technique is alsoapplicable to microchemical measurements and micromagnetic measurementsand is capable of being interfaced transparently with a normal forcesensing scanned probe microscope. The technology is also extendable intothe realm of making unique field emission tips.

The invention develops a new tip for such microelectrochemical and fastmicrothermocouple measurements, including those connected with thedetection of light and light induced heating effects, providing a tipthat can be interfaced with atomic force microscopy. In addition, withthe appropriate materials, the devices are applicable to micromagneticmeasurements. The techniques developed also are applicable to microfieldemission devices and yield such devices with improved characteristics.

The device of the invention consists, in one embodiment, of a tapered,nonconductive structure such as a glass or quartz micropipette having aconical tip. A metal wire is sealed within the micropipette and extendsto, and is exposed at, the end of the tip. The conical tip is coatedwith an electrically conductive or semiconductive layer that forms apoint thermocouple contact with the metal wire. The outer diameter ofthe tip is a few nanometers, with the diameter of the wire being 1/2 to1/3 the outer diameter of the tip.

The tip is fabricated by placing a metal wire within a tube ofborosilicate, for example, and the tube is heated and pulled to producetwo tapered micropipettes. Thereafter, a thin metal or semiconductorfilm is evaporated onto the outer surface of the tip, with the rate andtime of deposition being controlled to produce the desired thickness.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing, and additional objects, features and advantages of theinvention will be understood by those of skill in the art uponconsideration of the following detailed description of preferredembodiments thereof, taken with the accompanying drawings, in which:

FIG. 1 is a diagrammatic cross-sectional view of a curved micropipettefabricated in accordance with the present invention; and

FIG. 2 is an enlarged cross-sectional view of a micropipette tip for useas an extension to a conventional force sensing cantilever.

DESCRIPTION OF THE INVENTION

The invention represents a method and a device for the production ofmicrothermocouples for the measurement of temperature with resolutionsthat can be as good as submicron with response times that can be as goodas microseconds. The fabrication procedures result also inmicroelectrochemical devices that can have submicron resolution. All ofthe devices described herein can be effectively integrated intoconventional scanned probe microscopes.

The device of the present invention is a hollow micropipette 10constructed of glass or quartz or other nonconducting material 12. Themicropipette tapers inwardly along its length to form a conical tip 14,FIG. 1. The conical nonconductor has sealed in it a metal core, such asa wire 16 that extends through a hollow center 18 of the micropipette,with the terminal end 20 of the wire being coextensive with the terminalend 22 of the conical tip portion 14 of the micropipette 10. The metalwire is electrically conductive and is exposed at the micropipette tipto contact a metal or semiconductor layer 24 which coats the entire tip14. The layer 24 forms a point thermocouple contact with the exposed end20 of the metal wire at the tip.

This tip 14 can be a straight structure or it can be part of a bentglass micropipette (see FIG. 2), for the addition of normal forcesensing. The outer diameter 26 of tip 14 can be as small as a fewnanometers at the tip end 22, with the diameter 27 of the wire 16 insidebeing smaller by approximately 2-3 times.

For temperature measurements, the inner wire and the outer coating areconnected to a voltmeter (not shown). For temperature measurementswherein the structure is not distorted by heat transfer through the wirecore 16, the outside coating 24 surrounding the pipette 12 should have athickness 28 of at least an order of magnitude less than the thicknessof the coating 30 at the end 32 of the tip 14 when the coating is formetal. When the outer coating is a semiconductor this requirement is notimportant because of the low thermal conductivity of the semiconductor.

As illustrated in both FIG. 1 and FIG. 2, the wire 16 is generallyconical in shape at its end portion, with the diameter tapering inwardlyto its smallest size at its terminal end 20. The angle of taper, or thecone angle 34 of the wire near the end of the tip must be at least threetimes less than the ratio of the thermal conductivity of the mediumtimes the outer diameter 26 of the tip to the thermal conductivity ofthe inner wire material 16 times the diameter of this wire.

For microelectrode production the coating 32 on the front face of thetip can be eliminated, while for microchemical measurements both thiscoating 32 and the outer coating 24, can be eliminated. For fieldemission tips it may be necessary to etch the inner wire 16 either toexpose it or to recess it in the glass tip.

For micromagnetic measurements the metal wire 16 has to be composed of amagnetic material while the coating 24 has to be composed of anon-magnetic metal for shielding.

The technique for the production of these microelectrodes, fieldemission tips and microthermocouples involves placing a metal wire, from50-100 micrometers in diameter, inside a borosilicate tube with outerand inner diameters equal to 1.2 and 0.3 mm, respectively. Thistube-wire assembly is placed in a tube pulling device (not shown) whichallows the variation of 5 parameters: temperature of heating, the lengthof the segment heated, delay time between turning the heat on and thebeginning of pulling, the velocity of the pull and the strength of thepulling. Such a device can generate the tip shown in FIG. 1.

To produce the required structure a pulling operation is performed inthe following 2 steps:

First, the two ends of the glass tube and wire assembly are secured inthe pulling device and the glass tube is pulled (or stretched) to reduceits diameter. In this first step, the tube is pulled until its innerdiameter 18 is equal to the inner wire diameter. At this point the tubeis heated for several seconds without further pulling to ensure a firmconnection between the glass and the inner metal wire. Thereafter, thepulling is continued slowly, in 4 stages, resulting in the formation oftwo tapered glass-pipettes that are filled with metal to the end. Theshape of the pipette and wire cones and their diameters are determinedby the five pulling parameters noted above, with the wire core withinthe pipette reaching a diameter in the nanometer range.

This first step in the process is sufficient for producingmicroelectrodes. For field emission tips a second etching step issometimes necessary in order to either recess or to expose the innermetal wire.

Subsequently, a second thermoelectrode is made as a vacuum-evaporatedthin metal or semiconductor film 24. The deposition procedure controlsthe thickness of the film in various regions. Edge-like coatings with amaximal thickness of the coated layer near the tip can be produced tofulfill the conditions described above. Varying the rate and time ofdeposition provides coatings with the required thickness and electricalconductance [M. Adamov, B. Perovic, and T. Nenadovic, Thin Solid Films.24, 89 (1974)] to improve the spatial resolution of the thermocouples.

An alternate method to deposit the inner metal wire is anon-electrochemical method of metallic deposition such as self-assemblyof metal colloids on the inside glass or quartz surface 18 of thepipette 12 using technologies that have been described for suchdepositions on regular glass surfaces [R. G. Freeman et al Science 267,1629 (1995)]. Similar procedures could be employed to coat the outsideof the nonconducting structure and then this could be coupled with knownelectrochemical deposition techniques to eliminate some of thesubsequent deposition steps in producing these structures. In essencethis could, in principle, simplify the depositions required forachieving the devices described above.

Finally, to fabricate micromagnetic probes using this procedure, eitherof the above two techniques for deposition of a metallic wire inside isfeasible. Thus, this can be done by the pulling technology or thechemical methods noted above.

All of the above straight structures can be produced with the type ofcontrol that permits the resonance frequency of the structure to becontrolled to the point of permitting these straight structures to beused for monitoring lateral forces of surfaces. For such measurementswith these straight structures, modulations of the conical tip will haveto be imposed and the amplitude and the frequency of such modulationswill change when the tip approaches the surface.

For force measurements normal to a surface, the region above the sensingtip in the pipette like structure has to be bent. The method ofmicropipette bending is as follows. The micropipette is placed under alens of a microscope in such a way that a section of it that is a fewtens of micrometers distant from the tip is heated with a heat sourcesuch as a carbon dioxide laser also focused through a lens. As a resultof this heating the micropipette begins to soften in the irradiatedregion and this section of the micropipette eventually bends. The angleof bend can be controlled, and may approach, or even exceed, the 90°bend illustrated in FIG. 2. This procedure allows for fine control ofthe angle of bending and length of the resulting tip.

In distinction from the known wire thermocouples, the proposedconstruction allows creation of a submicron dimension microelectrode,field emission tip, micromagnetic tip, and a tip with a thermoelectriccontact. The spatial resolution that can be achieved can be as small asnanometers. Surface temperatures can be measured using the bentstructure shown in FIG. 2. Calculations and experiments show that theresponse time of the thermocouple with a tip diameter 26 of less than 1micron is about 1 microsecond. The method of fabrication of thesemicrothermocouples, microelectrodes and micromagnetic tips provides ahigh degree of control of the wire and glass diameter in the conicalpart of the tip, the cone angle of the conical structure, the resonantfrequency of the structure for its application to simultaneous forcemeasurements, etc.

A variety of applications with these devices and this technology is nowpossible. These include microthermal measurements, for example inmicrocircuits that are functioning, while simultaneously recording theirtopography with the force sensing capabilities of the tips.Micromagnetic measurements and combined micromagnetic, thermal andtopographic measurements can also be made. New and improved fieldemission tips for such areas as electron microscope are also provided,as are electrochemical measurements with microelectrodes that have highspatial resolution and the capability of simultaneously monitoringtopography with the force sensing capability of the tips.

Although the invention has been described in terms of preferredembodiments, variations and modifications will be apparent to those ofskill in the art.

What is claimed is:
 1. An assembly, comprising:an electricallynonconductive tube having a conical tip portion incorporating aninwardly tapering outer surface; a tapered electrically conductive innercore within said tube, said inner core having a terminal end coextensivewith said tube and tapering to a submicron diameter at an end portionthereof; and a conductive or semiconductive coating on the outer surfaceof said tip portion of said tube and surrounding the end portion of saidinner core, wherein the inwardly tapering outer surface of the innercore near a terminal end of the conical tip portion has a cone angle atleast three times less than the ratio of the thermal conductivity ofsaid coating times the outer diameter of the conical tip portion to thethermal conductivity of the material of said inner core times thediameter of the inner core.
 2. The assembly of claim 1, wherein saidtube includes a linear portion terminating in said tip portion, said tipextending from the linear portion with an angle that can approach ninetydegrees, resulting in a tapered structure with a bend leading to theconical tip.
 3. The assembly of claim 1, wherein said core is metalwire.
 4. The assembly of claim 1, wherein said tapered nonconductivetube is a silicon insulating material.
 5. The assembly of claim 1,wherein an applied voltage between said core and said coating produce apoint heat source at the end portion of said core.
 6. The assembly ofclaim 1, wherein said tube is a micropipette.
 7. An assembly,comprising:an electrically nonconductive tube having a conical tipportion incorporating an inwardly tapering outer surface which tapers toan outer submicron diameter; a tapered metal inner core within saidtube, said core having a terminal end coextensive with said tube, saidcore tapering to an outer diameter of 1/2 to 1/3 the outer diameter ofsaid tip; a conductive or semiconductive coating on the outer surface ofsaid tip portion of said tube and surrounding the end portion of saidmetal core; and wherein said assembly has a resonant frequency selectedfor use in force sensing.
 8. The assembly as in claim 7, wherein theinwardly tapering outer surface of the inner core near the end of thetip has a cone angle at least three times less than the ratio of thethermal conductivity of said coating times the outer diameter of the tipto the thermal conductivity of the material of said inner core times thediameter of the inner core.
 9. An assembly comprising:an electricallynonconductive tube having a conical tip portion incorporating aninwardly tapering outer surface which tapers to an outer submicrondiameter; a tapered electrically conductive inner core within said tube,said core having a terminal end coextensive with said tube, said innercore tapering to an outer diameter of 1/2 to 1/3 the outer diameter ofsaid conical tip portion; and wherein said assembly has a resonantfrequency selected for normal and lateral force sensing.
 10. Anassembly, comprising:an electrically nonconductive tube having a conicaltip portion incorporating an inwardly tapering outer surface; a taperedelectrically conductive inner core within said tube, said inner corehaving a terminal end coextensive with said tube, and tapering to asubmicron diameter at an end portion thereof; a conductive orsemiconductive coating on the outer surface of said tip portion of saidtube and surrounding the end portion of said inner core; and said metalinner core having a cone angle near the end of the conical tip portionwhich is at least three times less than the ratio of the thermalconductivity of the coating times the outer diameter of the conical tipportion to the thermal conductivity of the inner core times the diameterof the inner core.
 11. An assembly, comprisingan electricallynonconductive tube having a conical tip portion incorporating aninwardly tapering outer surface wherein said nonconductive tube is anonconducting glass having an inner surface surrounding a hollow core; atapered electrically conductive inner core within said hollow core ofsaid tube, said inner core having an end portion coextensive with saidconical tip portion and having an outer surface diameter taperinginwardly to a diameter in the submicron range at a terminal end thereof;and the hollow core of said nonconductive tube having an internaldiameter substantially equal to the diameter of the outer surface of theinner core with said inner surface of said tube being sealed to saidouter surface of said inner core, the assembly being usable as amicroelectrode for electrochemical and other measurements and as a fieldemission source.
 12. The assembly of claim 11, wherein said conical tubeincludes a linear portion terminating in said tip portion, said tipportion extending from the linear portion at an angle that can approachninety degrees, resulting in a tapered structure with a bend leading tothe conical tip portion.
 13. The assembly of claim 11, wherein said tubeand inner core have resonant frequencies selected for use in lateralforce sensing.
 14. The assembly as in claim 11, wherein said tube andinner core have resonant frequencies selected for use in normal andlateral force sensing.
 15. The assembly of claim 11, in which the innercore in the tip of the conical structure is a magnetic material.
 16. Theassembly of claim 15, wherein said tube includes a linear portionterminating in said tip portion, said tip portion extending from saidlinear portion at an angle that can approach ninety degrees, resultingin a tapered structure with a bend leading to the conical tip portion.17. The assembly of claim 15, wherein said tube and inner core haveresonant frequencies selected for use in force sensing.
 18. The assemblyof claim 16, wherein said tube and inner core have resonant frequenciesselected for use in normal and lateral force sensing.
 19. The assemblyof claim 11, in which the internal inner core in the tip of the conicalstructure is a magnetic material and the surrounding glass tube iscoated with a metallic material.
 20. The assembly of claim 19, whereinsaid conical tip portion extends from a linear portion of said tube atan angle that can approach ninety degrees to produce a tapered structurewith a bend leading to the conical tip.
 21. The assembly of claim 20,wherein said tube and said inner core have resonant frequencies selectedfor use in force sensing.
 22. The assembly of claim 20, wherein saidtube and said inner core have resonant frequencies selected for use innormal and lateral force sensing schemes.
 23. The assembly of claim 11,wherein said inner core is an internal metal wire and the surroundingnonconductive structure is coated with a metallic material.
 24. Theassembly of claim 21, wherein the conical tip portion extends from alinear portion of said tube at an angle that can approach ninety degreesto produce a tapered structure with a bend leading to the conical tip.25. The assembly of claim 24, wherein said inner tube and said core haveresonant frequencies selected for use in force sensing.
 26. The assemblyof claim 24, wherein said tube and said core have resonant frequenciesselected for use in normal and lateral force sensing.
 27. The assemblyof claim 23, wherein the internal metal wire in the tip of the conicalstructure is a magnetic material and the surrounding glass structureincluding the terminal end thereof is coated with another metallicmaterial which makes a point contact with the terminal end of themagnetic material to permit simultaneous micromagnetic and microthermalmeasurements.
 28. The assembly of claim 27, wherein the conical tipportion extends from a linear portion of said tube with an angle thatcan approach ninety degrees to produce a tapered structure with a bendleading to the conical tip.
 29. The assembly of claim 27, wherein thedevice has resonant frequencies selected to permit the device to be usedin force sensing.
 30. The assembly of claim 28, wherein the device hasresonant frequencies selected to permit it to be used in normal andlateral force sensing.